Crosslinked cellulosic nanofiltration membranes

ABSTRACT

The present invention relates to a nanofilter formed by using a porous ultrafiltration membrane as a precursor, and carefully controlling reaction conditions so as to maintain sufficient hydrophilic nature of the membrane while causing the pore structure to close to a nanofilter range (less than 400 Daltons). This produces a solvent stable cellulose nanofiltration membrane capable of operating at satisfactory flux in aqueous solutions, and being low binding to organic biomaterials.

CROSS REFERENCE TO RELATED APPLICATIONS

The present patent application claims the benefit of U.S. ProvisionalPatent Application No. 60/837,381, filed on Aug. 11, 2006 and is herebyincorporated by reference in it's entirety.

FIELD

The present invention relates to a solvent resistant membrane and amethod of making it. More particularly it relates to a solventnanofiltration membrane and method of making it.

BACKGROUND OF THE INVENTION

Nanofiltration (NF) membranes have retention characteristics in therange between ultrafiltration and reverse osmosis. Nanofiltrationmembranes are used to remove multivalent ions and small organicmolecules in the molecular weight range of approximately 200-1000Daltons. The ability to remove small organic molecules has led to muchinterest for applications in pharmaceutical industries. In particular,there is an interest in operating in organic solvent streams to separatesmall molecules such as synthetic antibiotics and peptides from organicsolutions. In these types of applications, a high permeability isrequired for economical operation.

Polar organic solvents, such as dipolar aprotic solvents, particularlysolvents such as N-methyl pyrrolidone (NMP), dimethylacetamide (DMAC),and dimethylsulfoxide (DMSO) are used as solvents or media for chemicalreactions to make pharmaceuticals and agrochemicals (for example,pyrethroid insecticides) industry. These powerful solvents will causesevere damage to commonly used polymeric membrane filters made frompolysulfone, polyethersulfone, polyacrylonitrile or polyvinylidenefluoride polymers.

In many applications, it would be useful for the membrane to operatewith aqueous mixtures of solvents or with both aqueous solutions andsolvent based solutions in series. For such uses, hydrophobic membranesare not useful as they have very low permeabilities for aqueoussolutions. Low aqueous permeability in hydrophobic NF membranes is shownin Advances in Solvent-Resistant Nanofiltration Membranes, Ann. N.Y.Acad. Sci. 984 159-177 2003.

A typical use for these membranes is to concentrate products in organicor aqueous/organic solutions prior to a crystallization step. In otherapplications, process operators are able to remove low molecular weightimpurities and salts by diafiltration, which cannot be done with anevaporation step. Operators are also able to exchange solvents duringthis type of filtration process. Nanofiltration of organic solutions canreplace vacuum flash evaporators or rotovaps, providing a lower capitalcost process.

In the processing of peptides and other low molecular weight organicsolutes, the capacity of the membrane to be non-binding is an importantattribute. Solutes bound to the membrane lower permeability and reduceyield by irreversibly holding solute. Cellulose is well-known as havinga very low binding surface for such molecules, whereas hydrophobic andthe typical polyamide NF membranes are known to be highly binding.

Cellulose is solvent stable, being soluble only in strong solvents suchas carbon disulfide and solutions of dimethylacetamide with lithiumchloride. When crosslinked, cellulose has even less tendency to swelland is therefore a good candidate for a solvent stable NF membrane. Todate no commercial membrane has been produced because of thedifficulties involved in making a porous NF membrane from cellulose.

Rendall, in U.S. Pat. No. 3,864,289, describes a process for thepreparation of a cellulosic semipermeable membrane from a formulationcontaining a cellulosic membrane material, a cellulosic crosslinkingagent and a blocking agent. The use of a “blocking agent” is undesirablebecause it adds unnecessary complexity to the process and an additionalchemical change to the nature of the membrane material. Such blockingagents can also add to undesirable extractable material which cancontaminate the purified permeate.

Wan, in U.S. Pat. No. 4,853,129, describes regenerated cellulosemembranes for separating organic liquids, such as ketone dewaxingsolvents from dewaxed oil. Reacting regenerated cellulose membranes witha bifunctional reagent results in improvement of the membrane'sselectivity in organic liquid separations applications. Wan states thatthe process also serves to reduce the hydrophilicity of the membranes,and that by use of the crosslinking agents no unreacted hydroxy groupsare left after reaction, nor are any hydroxy groups introduced by thecrosslinking agent. Such membranes would not be suitable for use withwater solutions, or solutions with appreciable amounts of water.

The membranes in the Wan patent have rejection values for oils ofmolecular weights in the range of from about 300-600 Daltons of 55%-90%.(Molecular weight data from concurrent U.S. Pat. No. 4,510,047.) Theserejections are not suitable for the high value added products ofpharmaceutical manufacturers.

Beer et al, in U.S. Pat. No. 5,739,316, claims a process for making across-linked cellulose hydrate ultrafiltration membrane comprisingcontacting a cellulose hydrate membrane with an aqueous alkalinesolution of a water soluble diepoxide. Besides being limited to watersoluble diepoxides, Beer teaches away from the use of organic solventsin the reactions as being technically difficult and expensive. Moreover,Beer states as an objective of his invention a process that does notmodify the high flux of the membrane. This means that the membranes soproduced would not have increased rejection from the initialultrafiltration membrane.

Charkoudian, in U.S. patent application Ser. Nos. 11/199491 and10/414988 teaches crosslinked and crosslinked and charged celluloseultrafiltration membranes that retain the ultrafiltration structure ofthe membranes.

U.S. Pat. No. 6,113,794 describes a nanofiltration composite membranecomprising a substrate ultrafiltration membrane formed fromnon-cross-linked ethylenically unsaturated nitrile polymer, and a porouscoating of a cross-linked hydrophilic polymer having a molecular weightof 20,000 to 2,000,000 and containing reactive functional groups, formedfrom an aqueous solution of the polymer containing 1.5-2.5% w/w of thepolymer. The patent is directed to chitosan coated membranes that arecompletely dried before being crosslinked. This will produce a densefilm rather than a porous membrane.

The U.S. Pat. No. 6,113,794 cannot be used with dipolar aprotic solventssuch as N-methyl pyrrolidone or dimethylacetamide because such solventswill dissolve the support layer and destroy the composite membrane.

Guo et al in Chinese Chemical Letters, Vol 5, (10) pp 869-872 1994reports on crosslinking large pore cellulose membranes with DMSO aqueousalkaline solutions of epoxyl propane chloride (epichlorohydrin). Thesemembranes were used for affinity separations. Such large pore membraneswould not be suitable for small molecule separations, and there is noteaching that the method could be used to make ultrafiltration or NFmembranes.

Several patent applications and articles have been published by authorsprimarily associated with GKSS Research Center. These all appear to bebased on the same technology. This method (WO 97/20622) coats asubstrate membrane with a low solids solution of cellulose-hydroxyether,such as hydroxyethylcellulose or hydroxypropyl cellulose, and thencrosslinks the coating with aldehyde, preferably a dialdehyde to thepoint of water insolubility. In an article in JAOCS Vol 76#1, pp 83-871999, Zwijnenberg et al report on nanofiltration of vegetable oils inacetone using composite membranes with a “cellulose-type top-layer.” InMembrane Technology #107 pp 5-8 1999, Ebert et al report onnanofiltration of vegetable oils in solvents with “cellulose-type”membranes in which the performance of the cellulose type membranes isinfluenced by the crosslinking conditions. Cellulose hydroxyethers are adifferent class of material from regenerated cellulose, as can be sen bythe referenced authors describing the material as cellulose-like.Cellulose hydroxyethers are water soluble and can be expected to behavedifferently from regenerated cellulose membranes in operation. Themembranes made from cellulose hydroxyethers are formed from thin denselayers and will have a different structure than the regeneratedcellulose made by phase separation methods.

The inventors of the present invention have found that by using a porousultrafiltration membrane as a precursor, and carefully controllingreaction conditions so as to maintain sufficient hydrophilic nature ofthe membrane, they can produce a solvent stable cellulose nanofiltrationmembrane capable of operating at satisfactory flux in aqueous solutions,including aqueous-solvent mixtures or blends, which is also low bindingto organic biomaterials.

SUMMARY OF THE INVENTION

The present invention is a cross-linked cellulose nanofiltrationmembrane capable of filtering solutes from organic solvents, includingdipolar aprotic solvents, aqueous solutions, and mixtures of water andorganic solvents.

The membrane comprises obtaining a preformed porous support capable ofoperation in dipolar aprotic solvents, forming a celluloseultrafiltration membrane on said support, and crosslinking the cellulosemembrane in a controlled manner so as to render the resultantnanofiltration membrane capable of retaining solutes of greater thanabout 200 Daltons.

A preferred embodiment of the membrane has a microporous membranecapable of operation in dipolar aprotic solvents as a support. A morepreferred embodiment has a microporous membrane support made fromultrahigh molecular weight polyethylene as a support.

In an embodiment, the cellulose ultrafiltration precursor membrane has amolecular weight cutoff of less than about 5000 Daltons.

In an embodiment, the nanofiltration membrane comprises a hydrophilicnanofiltration membrane comprising crosslinked cellulose capable ofoperation in dipolar aprotic solvents.

In an embodiment, the nanofiltration membranes comprise crosslinkedcellulose, capable of operation in dipolar aprotic solvents, saidmembrane comprising a cellulose ultrafiltration membrane reacted with amultifunctional crosslinking reagent through hydroxyl groups in theglucose units, under conditions whereby sufficient hydroxyl groups areleft unreacted to provide a hydrophilic membrane and whereby theresultant filter is a nanofilter.

In an embodiment, the nanofiltration membrane comprises hydrophilicnanofiltration membranes comprising crosslinked cellulose capable ofoperation in dipolar aprotic solvents, suitable for removing organicsolutes of greater than about 400 Daltons from organic solventsolutions.

In an embodiment, the invention comprises a method of removing organicsolutes from organic-aqueous or aqueous-organic solvent solutionscomprising passing the solution through a hydrophilic crosslinkedcellulose nanofiltration membrane, said membrane comprising a celluloseultrafiltration membrane capable of operation in dipolar aproticsolvents.

In an embodiment, the invention comprises a method of removing organicsolutes from organic-aqueous or aqueous-organic solvent solutionscomprising passing the solution through a crosslinked cellulosenanofiltration membrane, said membrane comprising a celluloseultrafiltration precursor membrane capable of operation in dipolaraprotic solvents, reacted with a multifunctional crosslinking reagentthrough hydroxyl groups in the glucose units, under conditions wherebysufficient hydroxyl groups are left unreacted to provide a hydrophilicmembrane and render the membrane a nanofilter.

In an embodiment, the crosslinked cellulose nanofiltration membraneshave controlled amounts of either negative or positive charge. Chargeadded to the membrane's internal and external surfaces has been shown toprovide improved retention of similarly charged molecules.

The membrane is used to remove or concentrate organic or inorganicsolutes of from about 200 kD to 1000 kD from the feed solution bysealing the membrane in a pressure holding device, such as a spiralwound module, a pleated cartridge, or a plate and frame type cassette,or other device, so that one side faces the higher pressure feed sideand the other side of the membrane is at the lower pressure permeatestream. A pressurized feed solution stream is introduced to thecellulose side of the membrane and the permeate liquid collected fromthe lower pressure downstream side of the membrane. Solutes greater thanabout 200 kD are retained (rejected) on the upstream side of themembrane. Lower molecular weight solutes, such as solvents pass throughthe membrane. Additionally, other low molecular weight solutes, such assalts and ions also pass through the nanofilter membrane providing aconcurrent desalting of the solute.

In some applications, after concentration of the desired solute, one ormore other solutes may be added to the upstream side to exchangesolvents. Also, pure solvent of the kind used in the original solutionmay be added to the concentrated solute to produce a purer solutionrelative to the original feed, now that low molecular impurities havebeen removed by passage through the membrane.

DETAILED DESCRIPTION OF THE INVENTION

The inventive nanofiltration membrane comprises a crosslinked cellulosicmembrane, preferably integral with a support layer. The cellulosicmembrane is made from a cellulosic ultrafiltration membrane preferablymade by the process of U.S. Pat. No. 5,522,991 (which is incorporatedherein by reference). The change in the membrane caused by thecrosslinking reaction is from a membrane that separates primarily bysize (ultrafiltration) to one where other interactions between the porematerials and the solutes play an important role in rejection ofsolutes. In nanofiltration, the forces on the solute molecule thatretard entrance of solute into the pores, or transport through theporous nature of the membrane derive from the nature of the membranematerial and the intimacy of solute to the pore surface. Charge anddielectric constant effects on the solute and solvent in the poresaffect transport through the pores in a manner absent in the largerpores of ultrafiltration membranes. Therefore, when making ananofiltration membrane by the process of the present invention, thepore size was reduced to obtain a porous structure in whichsolute-membrane material interactions were important, and whererejection simply by size was not the only mechanism.

Supports suitable for the present invention can be made from polymerssuch as polyethylene, polypropylene, or polyether-ether ketone (PEEK)capable of operation in dipolar aprotic (e.g. DMSO, DMF, NMP) solvents.Supports need to combine porosity for flow, mechanical strength andflexibility and resistance to swelling or dissolution by organicsolvents. Supports may be non-woven or woven fabrics made from, forexample, polyolefins, polyethylene terephtalate, or fluorinated polymerssuch as polytetrafluoroethylene. A preferred support is a microporousmembrane. A more preferred support is a microporous membrane made fromultrahigh molecular weight polyethylene (UPE) produced by the processdescribed in U.S. Pat. No. 4,778,601.

Cellulose membranes can be formed by immersion casting of a celluloseacetate or other cellulose ester polymer solution onto a support. Thecellulose ester is then hydrolyzed to cellulose by using a strong basesuch as 0.5N NaOH. A preferred method of making a celluloseultrafiltration membrane is described in U.S. Pat. No. 5,522,601 whereina solution of cellulose acetate is coated onto a UPE microporousmembrane and coagulated into a membrane. The membrane is then hydrolyzedwith sodium hydroxide to form the cellulose membrane.

Alternatively, cellulose can be dissolved in solutions of solvents suchas dimethylacetamide (DMAC) or N-methyl pyrrolidone (NMP) with theaddition of a salt such as lithium chloride. This cellulose solution canbe used to form the membrane and subsequently eliminate the need forbase hydrolysis.

Cellulose ultrafiltration hollow fiber membranes can be made by spinninga cuprammonium solution of cellulose into an acetone-water coagulatingsolution as described in U.S. Pat. No. 4,604,326. A similar process canbe used to produce flat sheet membranes.

These examples are not to limit the methods possible to make celluloseultrafiltration membranes, but as examples of some of the variousmethods available to a practitioner of this art.

Preferred cellulose membranes have molecular weight cutoff (MWCO) valuesof 10,000 Daltons (D) or lees, more preferred cellulose membranes haveMWCO less than 5000 D. A preferred membrane is PLCCC, produced byMillipore Corporation of Billerica, Mass.

Crosslinking comprises a heterogeneous reaction between the dissolvedcrosslinking reactant and a membrane. The reaction solution can be basedon an aqueous or an organic solvent, or an aqueous-organic mixture.Preferred organic solvents are N-methyl pyrrolidone, dimethyl acetamide,dimethyl sulfoxide, dimethyl formamide or similar solvents.

Typical crosslinkers are di- or multi-functional epoxides. Examples areepichlorohydrin, butandioldiglycidyl ether (BUDGE),ethylenedioldiglycidyl ether (EDGE), polyethyleneglycoldiglyciyl ethers,and butane diepoxide. Multifunctional N-methyl methoxy compounds mayalso be used as crosslinking reagents. Examples are Cymel 385 andPowderlink 1174, both available from Cytec Industries of West Patterson,N.J.

Crosslinker concentration in the reaction solution is about 5% by weightto about 60% by weight, with a preferred range of about 10% to about 40%by weight.

A skilled practitioner will determine the reaction based on reactiontemperature and reaction conditions. Generally, the reaction will takeplace at a faster rate at higher temperatures. A larger reaction vesselwill require more time to reach the reaction temperature and to cooldown. Higher pressures may be used to increase reaction rate. Dependingon the reaction vessel, the practitioner may use a continuous flow,stirred tank or other means to improve contact of reactants to themembrane and thereby control the reaction. Higher concentrations willincrease reaction rate. Crosslinker type as well as solvent will alsoplay a role in determining reaction time. Hydroxyl ion activity isanother important reaction condition.

Preferred reaction times are from about two to about one hundred hourswith preferred reaction times of about 4 to about 24 hours. The reactioncan be run at room temperature, and up to about 60° C., with preferredtemperatures being from 25° C. to about 50° C. One of ordinary skill inthe art will be able to modify or reduce this time by increasing forexample the mass transfer rates, by using a continuous web or by furtherincreasing reaction rates by adjusting temperature, concentrations andother like parameters.

When a multifunctional epoxy is used, the reaction is run at basicconditions. Sodium or potassium hydroxides are generally used. Typicallyabout 0.1M to about 1M hydroxide solutions are used. The skilledpractitioner will be able to balance the reaction against alkalinedeterioration of the cellulose. Higher hydroxide concentrations andhigher reaction temperatures will accelerate alkaline deterioration,lower concentrations of hydroxide and lower temperatures will slowdeterioration rate as well as crosslinking reaction rate.

Powderlink 1174, Cymel 385 and similar crosslinking agents(multifunctional N-methyl methoxy compounds) crosslink cellulose throughthe hydroxyls on the cellulose with an acid catalyst, such astoluenesulfonic acid. Other similar acid catalysts are organic sulfonicacids and non-oxidizing mineral acids. Weak or moderately acidconditions, of pH about 2 to 4, are appropriate. A preferred catalyst isCycat 4040, a sulfonic acid catalyst available from Cytec Industries.While more acidic conditions may increase the reaction rate, thepractitioner must take care not to cause acid deterioration of thecellulose membrane.

The reaction between the cellulose membrane and the crosslinkingreactants can be done in aqueous solutions, either 100% water or mixedwith solvents such as methylethyl ketone, methylpentanediol, acetone,other ketones. This list is not limiting. A skilled practitioner will beable to develop this method using convenient and solvents suitable totheir requirements.

The surface charge of the present invention can be made to have anegative charge, either through a one step or two step process. In theone step process, the charge modifying reactant is added to thecrosslinking solution. In the two step method, the charge addingreaction is conducted before or after the crosslinking reaction.

Suitable reactants for forming a negatively charged membrane includecompounds of the structure X(CH₂)_(m)A or alkali metals salts thereof. Xis a halogen, preferably chloride or bromide, A is carboxyl orsulfonate. Reaction time, reactant concentration, pH, and temperatureare used to control the amount of negative charge added to the surfacesof the membrane.

Positive charge may be imparted to the membrane by the use of glycidylquaternary ammonium compounds and quaternary ammonium alkyl halides.These molecules would have a structure of Y(CH₂)_(m)B where Y is ahalogen and B is a positively charge moiety.

In the present invention, the cross-linking reaction is preferably donebefore charging, i.e., adding charged groups to the membrane, because ascharged groups are added, charge repulsion between like-charged groupscauses polymer and membrane swelling, which can have a detrimentaleffect on membrane properties. It is possible to crosslink and addcharge simultaneously, if the crosslinking reaction is controlled at arate where the crosslinked membrane resulting can restrain potentialswelling by the added charge.

Practitioners will be able, with routine laboratory work, using theteachings herein, to produce nanofiltration membranes with the properbalance of pore size, charge and other material properties to have aworking and economically viable solvent resistant nanofiltrationmembrane.

Dextran Test

This test is based upon methods published by L. Zeman and M. Wales,“Separation Science and Technology” 16(3) p 275-390 (1981).

A sample of wetted membrane is placed in a test cell. A feed solutioncontaining a mixture of dextrans having nominal molecular weights from10,000 to 2,000,000 Daltons is contacted in a continuous flow mode or ina stirred cell with the upstream side of the membrane. The permeationrate is controlled by a peristaltic pump in order to run at low flux,and thereby eliminate concentration polarization on the feed side.

Samples of the permeate are examined for their molecular weightdistribution (MWD) by size exclusion chromatography. The resultingdistribution is compared to the MWD of the feed solution. A rejection atany elution volume can be calculated fromR₂=(h(f)_(v)−h(p)_(p))/h(f)_(v) where h(f)_(v) is the height of the feedsolution chromatograph at elution volume v, and h(p)_(v) is that of thepermeate solution at volume v. In this way a distribution of rejectionsas a function of v can be found. The relation between elution volume andmolecular weight of the solute is determined from the known MWD suppliedby the manufacturer of the dextran. Molecular size can be calculatedfrom the molecular weights by the relation of Granath and Kuist, J.Chromatography 28 p69-81 (1967). In this way a rejection vs. sizedistribution curve is generated.

A measure of MWCO is to determine the molecular weight at 90% rejection(R90) and consider that as the MWCO of the membrane.

Retention Test

Membrane discs were placed in a pressure cell having an inlet for thefeed stream and to allow pressure to be applied, and an outlet forpermeating liquid. Tests were done with dilute magnesium sulfate andraffinose solutions at 50 psi. Raffinose concentration in feed andpermeate were determined by HPLC with refractive index detection.Magnesium sulfate concentrations were determined by conductivity.

EXAMPLES A. Aqueous Solutions

PLCCC is a cellulose ultrafiltration membrane made by MilliporeCorporation of Billerica, Mass. It has a rated molecular weight cutoffas specified in the manufacturer's literature of 5000 Daltons.BUDGE is butanediol diglycidylether used as a crosslinking agent.EDGE is Ethylene glycol diglycidylether used as a crosslinking agent.

Example 1

A piece of PLCCC membrane 85 mm×165 mm, is treated with a solution of 40grams of ethyleneglycol diglycidylether(EDGE) dissolved in 60 grams 0.1MNaOH for 6 hours at 40 degrees C. by rolling the membrane with the EDGEsolution in a glass jar placed in a temperature controlled TECHNE HB1Dhybridizer. The membrane is washed three times with 200 cc of Milli-Q®water. It is stored until use in 200 cc of Milli-Q®) water containing0.01% sodium azide to prevent bacterial contamination. (Membrane #1).

Membrane 1 was tested for its rejection of neutral dextrans and itsbuffer flux according to the Dextran Test described above. The molecularweight at which 90% of the dextrans are rejected (R90) is tabulated inTable 1 along with the value for the PLCCC control. Also shown in Table1 is the buffer flux for Membrane 1 and the PLCCC control.

Membrane 1 was tested for its ability to reject MgSO4 according to theRetention Test described above. A 0.2% aqueous MgSO4 solution wasprepared and its conductivity measured to give 2.34 milliSiemens (mS).The membrane was placed in a high pressure filtration stirred cell. The0.2% feed solution was passed through Membrane 1 at a pressure of 50 psiand a stirring rate of 150 rpm. The conductivity of the filtrate was0.68 mS which corresponds to a concentration of 0.04% MgSO4. This is areduction of about 80% of the feed MgSO4. The flux of this membraneduring the nanofiltration experiment was 0.24 Imh/psi. This data isgiven in Table 2.

Membrane 1 was also tested for its ability to reject raffinose accordingto the Retention Test described above. Raffinose is a small sugarmolecule with a molecular weight of 594 Daltons. A 0.1% aqueousraffinose solution was prepared a processed using the same cell andsettings as for the MgSO4 experiment above. The feed solution and thefiltrate were analyzed for their raffinose concentration by HPLC using arefractiveindex (RI) detector. A 91% reduction in raffinoseconcentration was recorded after being processed with membrane 1. Thisdata is shown in Table 2.

B. Organic Solutions Example 2

A piece of PLCCC membrane, 85 mm×165 mm, is treated with a solution of20 grams of butanediol diglycidylether(BUDGE) dissolved in 40 gramsN-methylpyrrolidone and 40 grams of 0.5M NaOH for 90 hours at 24 degreesC. by rolling the membrane with the BUDGE solution in a glass jar placedin a temperature controlled hybridizer. The membrane is washed once with200 cc of methanol and twice with 200 cc of Milli-Q® water. It is storeduntil use in 200cc of Milli-Q® water containing 0.01% sodium azide toprevent bacterial contamination. (Membrane #2.)

Membrane 2 was tested for its rejection of neutral dextrans and itsbuffer flux according to the Dextran Test described above; these valuesare given in Table 1.

Membrane 2 was tested for its ability to reject raffinose in isopropylalcohol (IPA) according to the Retention Test described above. A 0.1%solution of raffinose in IPA was processed with Membrane 2 at 50 psi.After passing through Membrane 2, the concentration was reduced to0.029%. This is a decrease of 71% relative to the feed concentration.These data along with the flux and stir rate are shown in Table 3.

The dramatic difference in rejection seen for raffinose in DMAC comparedto isopropyl alcohol demonstrates that pore size alone does not controlrejection, but that the interactin of solute/membrane is important.

Example 3

A piece of PLCCC membrane, 85 mm×165 mm, is treated with a solution of40 grams of butanediol diglycidylether(BUDGE) dissolved in 40 gramsN-methylpyrrolidone and 40 grams of 0.5M NaOH for 40 hours at 24 degreesC. by rolling the membrane with the BUDGE solution in a glass jar placedin a temperature controlled hybridizer. After this period of time thissolution was discarded and a fresh solution with the same compositionwas introduced for 5 hours at 40 degrees C. The membrane is washed oncewith 200 cc of methanol and twice with 200 cc of Milli-Q® water. It isstored until use in 200 cc of Milli-Q® water containing 0.01% sodiumazide to prevent bacterial contamination. (Membrane #3).

Membrane 3 was tested for its rejection of neutral dextrans and itsbuffer flux according to the Dextran Test described above; these valuesare given in Table 1.

Membrane 3 was tested for its ability to reject raffinose inN-methylpyrollidone (NMP) according to the Retention Test describedabove. A 0.1% solution of raffinose in IPA was processed with Membrane 3at 50 psi. After passing through Membrane 3, the concentration wasreduced to below the detection limit of the HPLC equipment which is0.005%. This corresponds to a decrease of at least 95% relative to thefeed concentration. These data are listed in Table 3.

Example 4

A piece of PLCCC membrane, 85 mm×165 mm, is treated with a solution of40 grams of ethyleneglycol diglycidylether(EDGE) dissolved in 60 grams0.25M NaOH for 4 hours at 40 degrees C. by rolling the membrane with theEDGE solution in a glass jar placed in a temperature controlledhybridizer. The membrane is washed three times with 200 cc of Milli-Q®water. It is stored until use in 200 cc of Milli-Q water containing0.01% sodium azide to prevent bacterial contamination. (Membrane #4.)

Membrane 4 was tested for its rejection of neutral dextrans and itsbuffer flux according to the Dextran Test described above. The molecularweight at which 90% of the dextrans are rejected (R90) is tabulated inTable 1 along with the value for the PLCCC control. Also shown in Table1 is the buffer flux for Membrane 1 and the PLCCC control.

Membrane 4 was tested for its ability to reject raffinose inN-methylpyrollidone (NMP) according to the Retention Test describedabove. A 0.1% solution of raffinose in IPA was processed with Membrane 4at 50 psi. After passing through Membrane 4, the concentration wasreduced to below the detection limit of the HPLC equipment which is0.005%. This corresponds to a decrease of at least 95% relative to thefeed concentration. These data are listed in Table 3.

TABLE 1 Membrane Characteristics Before and After Crosslinking to FormNanofiltration Membrane Buffer Membrane cross R90 Flux Number linkingkDaltons lmh/psi PLCCC control 2645 3.8 1 EDGE 544 0.2 2 BUDGE 1113 0.23 BUDGE 886 0.1 4 EDGE 602 0.2

TABLE 2 Aqueous Nanofiltration % Before % After Membrane cross Nano-Nano- Number linking Feed filtration filtration % Reduction Flux(lmh/psi) PLCCC control 0.2% MgSO4 0.2 0.18 10 0.65 1 EDGE 0.2% MgSO40.2 0.04 80 0.2 PLCCC control 0.1% raffinose 0.1 0.08 20 0.65 1 EDGE0.1% raffinose 0.1 0.009 91 0.24

TABLE 3 Organic Solution Nanofiltration % Before % After Membrane crossNano- Nano- Number linking Feed filtration filtration % Reduction Flux(lmh/psi) PLCCC control 0.1% Raffinose/IPA 0.1 0.09 10 0.008 2 BUDGE0.1% Raffinose/IPA 0.1 0.01 90 0.008 3 BUDGE 0.1% Raffinose/NMP 0.1<0.005 >95 0.01 1 EDGE 0.1% Raffinose/NMP 0.1 <0.005 >95 0.01

Prophetic Charged Nanofiltration Membranes Example 1 Negatively Charged1

A piece of Membrane #1, 85 mm×165 mm, is treated with a solution of 22.5grams of bromopropylsulfonic acid sodium salt (BPSA) in 100 grams of0.5M NaOH for 4 hours at 25 degrees C. by rolling the membrane with theBPSA solution in a glass jar placed in a temperature controlledhybridizer. The membrane is washed three times with 200 cc of Milli-Q®water. It is stored until use in 200 cc of Milli-Q® water containing0.01% sodium azide to prevent bacterial contamination. (NegativelyCharged Membrane #1).

Example 2 Negatively Charged 2

The same BPSA reaction conditions as in Example 1 are employed exceptfor reaction time which is allowed to proceed to 16 hours. This producesa sulfonic acid modified membrane with a higher amount of negativecharge compared to 1 above. (Negatively Charged Membrane #2).

Example 3 Positive Charge 1

A piece of Membrane #1, 85 mm×165mm, is treated with a solution of 15grams of a 70-75% aqueous solution of glycidyltrimethylammonium chloride(GTMAC), 10 grams grams of 1M NaOH, and 75 grams of water for 3 hours at25 degrees C. by rolling the membrane with the GTMAC solution in a glassjar placed in a temperature controlled hybridizer. The membrane iswashed three times with 200 cc of Milli-Q® water. It is stored until usein 200 cc of Milli-Q® water containing 0.01% sodium azide to preventbacterial contamination. (Positively Charge Membrane #1.)

Example 4 Positive Charge 2

The same GTMAC reaction conditions as in Example 1 is employed exceptfor reaction time which was allowed to proceed to 16 hours. Thisproduces a quaternary ammonium modified membrane with a higher amount ofpositive charge compared to 1 above. (Positively Charged Membrane #2).

1) A hydrophilic nanofiltration membrane comprising crosslinkedcellulose capable of operation in polar organic solvents. 2) Themembrane of claim 1 wherein the polar organic solvent is a dipolaraprotic solvent. 3) The membrane of claim 1 wherein the polar organicsolvent comprises aqueous mixtures of polar organic solvents. 4) Themembrane of claim 1 wherein the dipolar aprotic organic solventcomprises aqueous mixtures of dipolar aprotic organic solvents. 5) Ananofiltration membrane comprising crosslinked cellulose, capable ofoperation in dipolar aprotic solvents, said membrane comprising acellulose ultrafiltration membrane reacted with a multifunctionalcrosslinking reagent through hydroxyl groups in the anhydroglucoseunits, under conditions whereby sufficient hydroxyl groups are leftunreacted to provide a hydrophilic membrane. 6) A hydrophilicnanofiltration membrane comprising crosslinked cellulose capable ofoperation in dipolar aprotic solvents, suitable for removing organicsolutes of greater than about 400 Daltons from organic solventsolutions. 7) The membrane of claim 1 further comprising a negativesurface charge chemically bonded to its surfaces. 8) The membrane ofclaim 5 further comprising a negative surface charge chemically bondedto its surfaces. 9) The membrane of claim 6 further comprising anegative surface charge chemically bonded to its surfaces. 10) Themembrane of claim 1 further comprising a positive surface chargechemically bonded to its surfaces. 11) The membrane of claim 5 furthercomprising a positive surface charge chemically bonded to its surfaces.12) The membrane of claim 6 further comprising a positive surface chargechemically bonded to its surfaces. 13) A method of removing organicsolutes from organic aqueous or aqueous-organic solvent solutionscomprising passing the solution through a hydrophilic crosslinkedcellulose nanofiltration membrane, said membrane comprising a celluloseultrafiltration membrane capable of operation in dipolar aproticsolvents. 14) A method of removing organic solutes from organic aqueousor aqueous-organic solvent solutions comprising passing the solutionthrough a crosslinked cellulose nanofiltration membrane, said membranecomprising a cellulose ultrafiltration membrane capable of operation indipolar aprotic solvents, reacted with a multifunctional crosslinkingreagent through hydroxyl groups in the anhydroglucose units, underconditions whereby sufficient hydroxyl groups are left unreacted toprovide a hydrophilic membrane. 15) The membrane of claim 1 furthercomprising the membrane has a support layer. 16) The membrane of claim 5further comprising the membrane has a support layer. 17) The membrane ofclaim 6 further comprising the membrane has a support layer. 18) Themembrane of claim 1 further comprising the membrane has a support layercapable of operation in dipolar aprotic solvents. 19) The membrane ofclaim 5 further comprising the membrane has a support layer capable ofoperation in dipolar aprotic solvents. 20) The membrane of claim 6further comprising the membrane has a support layer capable of operationin dipolar aprotic solvents. 21) The membrane of claim 1 furthercomprising the membrane has a support layer formed of a microporousmembrane. 22) The membrane of claim 5 further comprising the membranehas a support layer formed of a microporous membrane. 23) The membraneof claim 6 further comprising the membrane has a support layer formed ofa microporous membrane. 24) The membrane of claim 1 further comprisingthe membrane has a support layer formed of a microporous membrane andwherein the microporous membrane is comprised of ultrahigh molecularweight polyethylene. 25) The membrane of claim 5 further comprising themembrane has a support layer formed of a microporous membrane andwherein the microporous membrane is comprised of ultrahigh molecularweight polyethylene. 26) The membrane of claim 6 further comprising themembrane has a support layer formed of a microporous membrane andwherein the microporous membrane is comprised of ultrahigh molecularweight polyethylene.